A quantitative analysis of hydrogen production efficiency of the extreme thermophile Caldicellulosiruptor owensensis OL T Ahmad A. Zeidan*, Ed W.J. van Niel Department of Applied Microbiology, Lund University, P.O. Box 124, SE-221 00 Lund, Sweden article info Article history: Received 20 October 2009 Received in revised form 18 November 2009 Accepted 21 November 2009 Available online 16 December 2009 Keywords: Biohydrogen Extreme thermophile Caldicellulosiruptor owensensis Batch mode H 2 partial pressure Lactate formation Defined medium abstract Caldicellulosiruptor owensensis strain OL T (DSM 13100) is an obligately anaerobic, extreme thermophilic bacterium that is capable of utilizing a broad range of carbohydrates and producing H 2 as a metabolic by-product. The performance of C. owensensis on glucose and xylose was analyzed in lab-scale bioreactors to assess its potential use in biohydrogen production. Acetate, H 2 , and CO 2 were the main end products during exponential growth of the organism on either sugar. Lactate production was triggered during the transition into the stationary phase and was associated with an increase in the levels of specific L-lactate dehydrogenase activity. In addition, minor amounts of ethanol and propionate could be detected. H 2 and acetate yields were lower on xylose than on glucose, marking an opposite trend to biomass and lactate yields. The influence of elevated H 2 partial pressure on product distribution was more dramatic in xylose-fermenting cultures. Replacement of yeast extract in the medium with a standard vitamins solution improved H 2 yield on both sugars, where it reached 100% of the theoretical maximum, i.e. 4 mol per mol hexose, on glucose. By using the defined medium, both the maximum specific growth rate and the maximum volumetric H 2 production rate of C. owensensis increased significantly on glucose and almost doubled on xylose. Screening other sugars besides glucose and xylose revealed a clear sugar-dependent product-distribution pattern and a direct correlation between biomass and lactate yields, which might be explained considering energy metabolism of the cells. The organism is proposed as a new candidate for biohydrogen production at high yields. ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. 1. Introduction Hydrogen production from renewable resources is one of the inevitable avenues to a fully sustainable hydrogen economy. A wide variety of facultative and obligate anaer- obes are capable of producing H 2 through fermentation of carbohydrate-rich feedstock, presenting an approach of increasing interest for biohydrogen production. In this process of dark fermentation, molecular H 2 is produced in the oxidation of sugar molecules, as a means of recycling the electron carriers [1,2]. Based on thermodynamic constraints, maximally four molecules of H 2 can be obtained when a glucose molecule is oxidized exclusively to acetate and CO 2 [3]: C 6 H 12 O 6 þ 2H 2 O / 2 CH 3 COOH þ 2 CO 2 þ 4H 2 (1) which is only possible when the H 2 partial pressure ðP H2 Þ is kept adequately low [2]. In practice, the yield of substrate conversion to H 2 varies significantly according to the nature * Corresponding author. Tel.: þ46 46 2229875; fax: þ46 46 2224203. E-mail address: ahmad.zeidan@tmb.lth.se (A.A. Zeidan). Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/he international journal of hydrogen energy 35 (2010) 1128–1137 0360-3199/$ – see front matter ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.11.082